1. Field of the Invention
This invention relates to a cutting tool assembly having a composite sintered body as its cutter.
2. Description of the Related Art
Hard sintered bodies primarily comprising high pressure phase boron nitride or diamond are widely used as the cutting elements of cutting tools because of their high hardness and high thermal conductivity. These cutter elements are typically soldered on a base made of cemented carbides or hard alloys such as cermet. However, since such cutter elements have poor wettability against solders, they are usually combined with a bonding layer into a form of composite cutter body that can be soldered onto the base through the bond layer.
Soldering is typically used because if high pressure phase boron nitride or diamond cutting materials are subjected to high temperatures (such as during welding), the former is adversely converted into a low pressure phase hexagonal type boron nitride and the later into a low pressure phase graphite with reduced hardness. Therefore, they lose their usefulness as cutting instruments On the other hand, soldering does not have a very high bond strength. Since the price of such a cutting tool assembly is proportional to that of the cutting part, it is economical to minimize the size of the cutting part. However, since the solder has a relatively low bond strength, the cutting tool assembly is not usable unless the composite cutter has at least a predetermined area.
A typical prior art cutting tool assembly is shown in FIGS. 22 and 28 The cutting assembly includes a base 11 and a composite cutting part 14. The base is made of a cemented carbide or a hard alloy and has a notch at one corner thereof. The cutting part 14 includes a hard sintered body 12 and a support of cemeted carbide 18. The cutting part 14 is secured in the notch. This assembly is of a so-called throw away tip which is never reused by sharpening. Rather it is thrown away when the edge thereof is worn.
In the cutting tool shown in FIGS. 22 and 23, however, the stress parallel to the bond faces of the base 11 and the cutting part 14 (hereinafter referred to as feed force) during cutting lowers the bond strength that secures the cutting part 14 onto the base 11. Therefor, the cutting part 14 is liable to come off the base 11 or to be chipped.
Japanese Unexamined Patent Publication No. 292/1979 discloses a cutting tool assembly, wherein a hard polycrystalline substance is used as the cutting part. The thickness of the cutting face of the cutting part (between its top and bottom surfaces) is greater than the width (between the side surfaces) where it is secured to the base. In this cutting tool assembly, however, the cutting part is secured to the base merely by soldering a very limited bonding area. Therefore, it suffers the disadvantage of low resistance against the feed force.
The cutting tool assembly disclosed in Japanese Unexamined Patent Publication No. 78391/1979 has a cutting part comprising a strip-like hard sintered body in which one pair of opposing sides are longer than the remaining pair. The sintered body is embedded on the cutting face such that one of the shorter sides serves as the cutting edge. In this cutting tool assembly, however, when the strip-like sintered body is soldered onto the base, it often cracks due to the influence of thermal stress. Moreover, repeated heat treatment for repositioning the sintered body to provide new cutting edges causes transition of the high pressure phase boron nitride or diamond into a low pressure stable state with low hardness.
Japanese Utility Model Publication No. 10882/1988 also discloses a composite disposable tip assembly cutting tool. As shown in FIG. 24, the cutting part 14 comprises upper and lower sintered body layers 12 and an intermediate layer of cemented carbide support 18. The cutting part is secured at one corner of the base 11 over the entire thickness thereof. However, the cutting part 14 is merely secured at the support of cemented carbide 18 by soldering. Therefore, the retention of the hard sintered body 12 against the feed force and the stress applied perpendicularly to the cutting face (hereinafter referred to as cutting force) will be insufficient.
Additionally, when processing of metal or the like is subjected to cutting, insufficient discharge of chips will damage the cutting tool or cause the surface of the work to be poorly finished. Additionally, in an automatic machining center, productivity is dependent upon the efficient disposal of chips.
In order to discharge chips effectively, it can be contemplated to vary cutting conditions or to provide a chip breaker adjacent to the cutting part of the cutting tool for breaking the chips into adequate length of pieces. The chip breaker comprises a groove or a protrusion where chips are broken into finer pieces. In the former method, the acceptable range in which the cutting conditions can be varied will be quite limited under circumstances where high accuracy and high efficiency in cutting are desired. Therefore, the latter method is generally preferred because it allows the formation of various shapes of grooves and protrusions and has wide applications.
When a cutting tool is provided with a chip breaker, the chip breaker should have high fracture toughness, excellent abrasion resistance and an ability of allowing relatively easy cutting in order to exhibit its function effectively. Thus, a few embodiments of chip breakers for throw away tips made of cemented carbide are shown in FIGS. 26 to 29.
As shown in FIGS. 26 and 27, a throw away tip 15 having a triangule is notched along the respective sides to form chip breakers 16. Whereas as shown in FIGS. 28 and 29, a throw away tip 15 is notched along the entire periphery to form a chip breaker 16. In a cutting tool made of a sintered hard metal, such chip breaker 16 can be formed relatively easily.
However, it is difficult to form a chip breaker for chip disposal on the composite sintered body due to its high hardness. Soldering a chip breaker onto the cutter assembly is also difficult due to its thinness (e.g. usually not more than 1 mm).
Another prior art cutting tool assembly has a disposable cutting tip assembly as shown in FIG. 25. A chip breaker piece 17 is merely held on the disposable tip assembly 15 by the downward force from a support 18. Therefore, this cutting tool assembly suffers the drawback that the breaker piece 17 can slip or even be lost when the tip assembly 15 is replaced.